1. Field of the Invention
The present invention relates to a process for producing an Fe--B--R based permanent magnet having an excellent corrosion-resistant film. More particularly the present invention relates to a process for producing an Fe--B--R based permanent magnet which has, on its surface, an excellent corrosion-resistant film having an excellent adhesion to the surface of the magnet and capable of being formed easily and at a low cost without carrying-out of a plating treatment and a treatment using hexa-valent chromium, and which can exhibit a stable high magnetic characteristic that cannot be degraded even if the magnet is left to stand under high-temperature and high-humidity conditions of a temperature of 80.degree. C. and a relative humidity of 90%.
2. Description of the Related Art
An Fe--B--R based permanent magnet, of which an Fe--B--Nd based permanent magnet is representative, is practically used in various applications, because it is produced of an inexpensive material rich in natural resources and has a high magnetic characteristic.
However, the Fe--B--R based permanent magnet is liable to be corroded by oxidation in the atmosphere, because it contains highly reactive R and Fe. When the Fe--B--R based permanent magnet is used without being subjected to any treatment, the corrosion of the magnet is advanced from its surface due to the presence of a small amount of acid, alkali and/or water to produce rust, thereby bringing about the degradation and dispersion of the magnetic characteristic. Further, when the magnet having the rust produced therein is assembled into a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute surrounding parts or components.
There is an already proposed magnet which has a corrosion-resistant metal-plated film on its surface, which is formed by a wet plating process such as an electroless plating process and an electroplating process in order to improve the corrosion resistance of the Fe--B--R based permanent magnet with the above-described point in view (see Japanese Patent Publication No. 3-74012). In this process, however, an acidic or alkaline solution used in a pretreatment prior to the plating treatment may remain in pores in the magnet, whereby the magnet may be corroded with the passage of time in some cases. In addition, the magnet is pooi in resistance to chemicals and for this reason, the surface of the magnet may be corroded during the plating treatment. Further, even if the metal-plated film is formed on the surface of the magnet, as described above, if the magnet is subjected to a corrosion test under conditions of a temperature of 60.degree. C. and a relative humidity of 90%, the magnetic characteristic of the magnet may be degraded by 10% or more from an initial value after lapse of 100 hours.
There is also a conventionally proposed process in which a corrosion-resistant film such as a phosphate film or a chromate film is formed on the surface of an Fe--B--R based permanent magnet (see Japanese Patent Publication No. 4-22008). The film formed in this process is excellent in adhesion to the surface of the magnet, but if it is subjected to a corrosion test under conditions of a temperature of 60.degree. C. and a relative humidity of 90%, the magnetic characteristic of the magnet may be degraded by 10% or more from an initial value after lapse of 300 hours.
In a process conventionally proposed in order to improve the corrosion resistance of the Fe--B--R based permanent magnet, i.e., in a so-called aluminum-chromate treating process (see Japanese Patent Publication No. 6-66173), a chromate treatment is carried out after formation of an aluminum film by a vapor deposition process. This process remarkably improves the corrosion resistance of the magnet. However, the chromate treatment used in this process uses hexa-valent chromium which is undesirable for the environment and for this reason, a waste-liquid treating process is complicated. It is feared that a film formed in this process influences a human body during handling of the magnet, because it contains just a small amount of hexa-valent chromium.
On the other hand, there is a conventionally proposed process in which a primary coat layer is formed of a metal used as a main component on the surface of an Fe--B--R based permanent magnet and a glass layer is formed on the surface of the primary coat layer (see Japanese Patent Application Laid-open No. 1-165105). If the primary coat layer is formed using a wet plating, the magnet may be corroded with the passage of time, as described above. For example, if the primary coat layer is formed by a vapor deposition process such as a vacuum evaporation process, it is possible to provide a magnet free of such a problem and having an excellent corrosion resistance. However, to conduct the vapor deposition process, a large-sized device is required and moreover, this device is expensive. A cleaning treatment for the surface of the magnet is required as a pretreatment, and to form the primary coat layer formed of an easily oxidized metal used as a main component such as aluminum, tin, zinc and the like, an extremely high vacuum degree is required. For this reason, an evacuating treatment for a long period of time is required, and thus, the complication of the producing process and the prolongation of the time required for the producing process cannot be avoided.